Camouflage Using Surface Disruption: The Importance of Corners Versus Edges.

Disruptive colouration is a common mode of camouflage used by predators and prey to conceal their body contours. However, it is unclear how disruptive colouration hinders the detection and recognition of three-dimensional (3D) body shapes. In human visual systems, corners, not edges, of an object are critical for shape processing and contour completion. However, whether corners are also critical for 3D shape recognition in non-human animals has not been investigated. To test this, in Experiment 1 we presented 3D-printed moth-like targets with either corner disruption, edge disruption or no disruption to wild, free-living birds. We repeated this in Experiment 2 with 2D targets, comprising images matched to Experiment 1, to determine if pictorial cues (patterns that might produce an impression of depth) had the same effect. Contrary to our predictions, we found no effect of surface disruption location on survival probability for either 3D or 2D moth targets. In contrast to previous work, we found that targets with disrupted surfaces, irrespective of the disruption location, did not have higher survival than those with continuous surfaces. All targets were designed to match the background, so perhaps there were no further camouflage benefits of surface disruption. However, when the data from both experiments were combined, we found that 3D targets had higher survival than 2D targets, perhaps because moths are typically flattened, so this shape is less familiar to the birds. Most of the variation in our data was explained by the spatial and temporal structure of the data, owing to different predator communities and seasonal variation in predation risk. Given corners are critical for shape recognition in humans, developing an appropriate system to test visual perception in non-human animals will provide key insights into the role of visual perception in the predator-prey arms race.